Historically, there have been numerous attempts to obtain a relatively inexpensive ceramic material which exhibits desirable properties such as resistance to thermal shock, ability to function as a thermal insulating material, high mechanical strength, low coefficient of thermal expansion, the ability to form the ceramic material to a net or near net shape, the capability of achieving little or no shrinkage upon firing (e.g., sintering) and the ability to produce economically such a body. It is a difficult engineering task to achieve a body containing a)i of the aforementioned desirable properties. To date, the challenge has not been adequately met.
A first practical application for,a ceramic body which possess the above-discussed properties is the use of the body as a thermally insulating tube or shape within a metal body. For example, in applications where it is necessary for hot gasses to flow through a metal body, a ceramic tube or channel may be used as a liner which is encased within the metal body, thereby defining a channel for the flow of hot gasses therethrough. Such applications require that the ceramic article possess adequate heat insulating properties.
A practical and inexpensive method for forming a composite body having an integral ceramic surrounded by a mass of metal entails solidifying a cast molten metal around a ceramic article. However, the ceramic body often cracks due to thermal shock which occurs during casting. Further, when the cast metal solidifies and cools around the ceramic article, contraction of the surrounding metal can occur such that high compressive stresses may result in the ceramic article which also may result in failure of the ceramic. Particularly, the thermal expansion coefficients of the ceramic and the metal typically differ from each other such that the stresses which are exerted upon the ceramic article can result in crack initiation and/or catastrophic failure of the ceramic. Such crack initiation and/or failure has been especially pronounced in low strength, hollow, ceramic articles. Moreover, crack initiation and/or failure in the casting metal has also been a problem in certain applications. For example, when the metal surrounding the ceramic is thin, the greater magnitude of contraction of the metal during cooling can result in tensile stresses in the metal which can lead to a yielding or failure thereof.
One technique known in the art for ameliorating the undesirable stresses involved requires the use of ceramic articles having relatively thick, porous coatings or layers of material. However, ceramic-metal composite bodies which employ thick coatings on a ceramic article may be prone to physical damage due to the presence of a relatively thick and weak layer between the metal and the ceramic. Moreover, such coatings can be difficult, and in certain cases expensive, to apply. Still further, in some applications the presence of a coating may be completely unacceptable. Moreover, a requirement for specific mechanical properties in a ceramic may reduce the capacity to deliver desirable thermal properties.
A specific application which involves placing a ceramic article within a mass of metal is an exhaust port for an engine (e.g., an internal combustion engine). Specifically, a ceramic article which can be surrounded by molten metal in a casting operation, (e.g., surrounded by molten metals such as aluminum and iron) without resulting in substantial injury to the ceramic or the metal which has been cast and cooled, would be advantageous in production of articles such as an automotive exhaust port liner.
A need therefore exists to provide an inexpensive, reliable material composition for ensuring that ceramic articles will survive the stresses associated with metal casting operations so as to provide structurally sound ceramic-metal composite bodies. In particular, a need exists for ensuring that molten metal may be cast around a ceramic article without degrading the mechanical properties of the ceramic and without degrading the mechanical properties of the ceramic-metal composite or assembly. In addition, a need exists to ensure that when molten metal is cast around a ceramic article and the thickness of the cooling metal is thin relative to the thickness of the ceramic article, and/or the tensile strength of the metal is low compared to the compressive strength of the ceramic, that the metal will not crack due to the development of tensile stresses therein.
Another practical application for a ceramic composite material which exhibits the above-discussed mechanical properties is the use of the material in a turbine engine shroud. The shroud is the nonrotating cylindrical assembly which surrounds the tips of the turbine blades. The environment that a turbine engine shroud is subjected to is one which requires a body to be thermally insulating, have a high thermal shock resistance, have a low coefficient of thermal expansion, etc. Moreover, in some instances the turbine blades of a turbine engine may expand due to thermal energy and contact (e.g., rub against) the turbine engine shroud. For example, during initial operation of a turbine engine, the turbine blades, in some cases, are designed so that they will contact the shroud. This intentional contacting is effected so that the shroud will be abraded or machined by the tips of turbine blades such that the clearance between the blade tips and the shroud is minimized. By minimizing such clearance, the undesirable bypass of working fluid is minimized. When such contact occurs, the turbine blades of the engine could be damaged and catastrophically fail (i.e., break). If the turbine blades were damaged, it is possible that performance of the engine could be affected adversely, or it is possible that a more catastrophic failure of the turbine blades could result (e.g., the engine could be destroyed). Thus, engineers have been faced with the problem of contact of turbine blades with the engine shroud, such contact leading to potential failure of the turbine engine if the shroud is not readily machinable or abradable by the blade tips. Accordingly, a need exists to provide an improved material which exhibits all the above-discussed properties. In other words, the material should be capable of surviving in a turbine engine; and when rotating blades of the turbine contact the engine shroud, the engine shroud is machined by the blades and neither of the engine shroud or the turbine blades are adversely affected by such machining.